This invention relates to a device for use with a key for a cylinder lock to indicate the last operation performed on the lock by the key, thus indicating whether the lock is in the locked or unlocked position. If, after leaving premises, a person having primary responsibility for locking the premises is uncertain whether the lock was secured, the status of the lock can be ascertained by a glance at the key without the need to return to the premises.
Devices have been suggested having the object of indicating the status of a lock. These have suffered from the disadvantages of including small moving parts, such as small springs, or having complicated construction. A further disadvantage with previous devices is that they are only capable of giving an accurate indication of the status of a lock if used with extreme care, particularly with precision in turning the key in the lock, and are not adapted to maintain their accuracy where used in more realistically predictable conditions of possible slight over-rotation in the lock, attempts to insert or remove the key at a position slightly out of the correct alignment, or somewhat rougher than ideal handling.